1. Field of the Invention
The present invention relates to a ground fog and ground removing system for a color image forming apparatus that produces an image by converting color-separated signals B, G, R into colorant recording signals Y, M, C, K through a color conversion means.
2. Related Art
A digital copying machine converts an analog signal read from a document into multi-valued digital data, subjects the converted data to image quality adjustment such as for granularity, tone characteristic, definition, etc., and records and reproduces an image with dots. The use of the multi-valued digital data allows not only image data processing producing a high-definition, well reproduced image, but also various correcting and editing functions using such data in combination with a memory to be performed with ease.
Also, a full-color digital copying machine reads a document optically, color-converts by correction the color-separated read signals B (blue), G (green), R (red) into recording signals Y (yellow), M (magenta), C (cyan) for colorant such as toner, ink, or ink doner film, and reproduces the full-color document by generally superposing the dotted images formed by the respective colorants one upon another. In this case, since the image formed by using the colorants in equivalent quantities turns achromatic, a technique for removing the recording signal components Y, M, C in equivalent quantities (UCR: under color removal) is employed to avoid wasteful consumption of the colorants. However, the UCR process, reducing the consumption of the colorants, impairs depth and thickness in each produced color, thus disadvantageously giving the reproduced color image an impression of scarce voluminosity as a whole. In addition, reproduction of gray and black being incompatible with reproduction of colors of high chroma, a simple UCR process cannot adequately improve color reproducibility. To supplement such insufficient voluminosity in color reproduction or to produce gray, black or Chinese ink (K) is generated so as to correspond to the quantities of colorants to be subjected to under color removal. A summary of a digital color image forming apparatus as disclosed, for example, in Unexamined Japanese Patent Publication No. Hei. 2-70173 proposed by the applicant will hereunder be described.
FIG. 6 is a diagram showing an exemplary configuration of a digital color image forming apparatus.
In FIG. 6, an IIT (image input terminal) 100 reads a document using CCD sensors and converts the read color-separated signals B, G, R into digital image data; and an IOT (image output terminal) 115 reproduces a color image by subjecting the image data to an exposing process by a laser beam and to a developing process. Components from an END (equivalent neutral density) conversion circuit 101 to an IOT interface 110 interposed between IIT 100 and IOT 115 constitute an image data edit processing system (IPS, or image processing system), which converts the read signals B, G, R into toner recording signals Y, M, C, and a recording signal K, and selects and outputs a recording signal corresponding to a developing color every developing cycle. Here, when converting the read signals (B, G, R signals) into the recording signals (Y, M, C, K signals), what matters is how to achieve color balance adjustment, color reproduction matching the input characteristic of the IIT and the output characteristic of the IOT, density and contrast balance adjustment, edge enhancement, dullness and moire adjustment, etc.
IIT 100 reads a pixel by the unit of 16 dots/mm for the read signals B, G, R using the CCD sensors, and outputs the read pixel in 24 bit-data (3 colors.times.8 bits in 256 tone levels). Each of the CCD sensors, covered with filters respectively corresponding to B, G, R on their surfaces, is 300 mm long at a density of 16 dots/mm, and scans 16 lines/mm at a process speed of 190.5 mm/sec, thereby outputting the read data at a speed of about 15M pixels/sec every color. IIT 100 converts analog data of B, G, R pixels into logarithmic data to thereby convert reflectance data into density data, and further converts the density data into digital data.
The IPS is provided to receive the read signals B, G, R from IIT 100, subject the received signals to various kinds of data processing to improve reproducibility or the like in color, tone, and definition, select a development process color recording signal out of the recording signals Y, M, C, K, convert the selected signal into an on/off signal, and apply the converted signal to IOT 115.
As shown in FIG. 6, the IPS consists of edit control modules including: an END conversion module 101 that adjusts (converts) data into a gray-balanced color signal; a color masking module 102 that converts the read signals B, G, R into recording signals Y, M, C corresponding to respective toner quantities by subjecting the B, G, R signals to matrix calculation; a document size detection module 103 that detects the size of a document at the time of prescanning and deletes a platen color (frame) during scanning; a color conversion module 104 that converts a color specified in a predetermined area in accordance with an area signal inputted from an area image control module; a UCR and black generation module 105 that not only generates such an appropriate quantity of K as not to cause turbidity in a produced color and reduces Y, M, C equally so as to correspond to the quantity of the generated K, but also gates the K signal as well as the under color-removed Y, M, C signals in accordance with signals in a monochromatic mode and in a full-color mode; a space filter 106 that performs the function of recovering dullness and removing moires; a TRC (tone reproduction control) module 107 that performs such processing as density and contrast adjustment to improve reproducibility, reversion between negative and positive images, and color balance adjustment; a reduction and enlargement module 108 that performs reduction and enlargement processing in the main scanning direction; a screen generator 109 that converts a toner signal expressed in process color tone into an on/off binary-coded toner signal and outputs such binary-coded toner signal; an IOT interface module 110; an area image control module 111 that includes an area generating circuit and a switch matrix; an area command memory 112; a color pallet video switch circuit 113; and a font buffer 114.
The conventional digital color image forming apparatus produces colorant recording signals Y, M, C, which are the subtractive primaries (in printers), from the color-separated read signals B, G, R, and various basic parameters involved in the course of preparing the colorant recording signals Y, M, C are fixed and they are determined based on a reference input-to-output correspondence. Thus, any poorly reproduced colors are usually adjusted in accordance with the user's preference using the tone adjustment function.
For a document consisting essentially of characters on a white ground, if colored paper, newspaper, straw paper, reproduced paper, or the like is used, it sometimes happens that the density of such paper itself appears as fog, impairing the quality of a reproduced image. To overcome this problem, a ground removal technique or the like involving the steps of obtaining a histogram by sample scanning and deliberately detecting the density of the ground or the like is adopted, as disclosed, for example, in Japanese Patent Application No. Hei. 2-145104 or Unexamined Japanese Patent Publication No. Hei. 2-224466, when making a black and white copy. When making a color copy, no ground removal is usually effected since highlights that are close to the background color must be reproduced well.
However, some documents to be color-separated use thick paper that is relatively opaque and some use thin paper that is relatively transmissive, and depending on the quality of paper as well as the platen cover lining material of the IIT, ground fog may, in some cases, show in greater degree than with ordinary paper.
For example, a white platen cover lining, with its high reflectance, causes irregular reflection even over the surface of a thin-sheet document that is relatively transmissive, thus exhibiting little reduction in reflectance. The same applies to the mirror surface platen cover lining such as one employed in the conventional digital color image forming apparatus. One of the reasons why the mirror surface is employed is that reflection by a single mirror body allows a white-ground document to be sensed easily in a later process with the document having been inputted at a low reflectance at the IIT.
However, if an automatic document feeder or the like is installed, a large friction coefficient is required to forward the sheet, which permits use of no mirror surface-finished cover of ordinary type. It is why a rubber belt is usually employed. However, since the rubber belt does not provide the benefits of the mirror surface, the user must choose between no document sensing using a white belt and the ground fog present on a thin sheet that is relatively easy to transmit light using a low reflectance belt. Otherwise, he must take some measure against ground fog present on the thin sheet that is relatively transmissive.
When a thin sheet document which is relatively transmissive and which is printed on both sides is read using the platen cover lining that is either white or mirror surface-finished, the image printed on the back side may, in some cases, be read together with the image on the front side. In contrast thereto, a low-reflectance type platen cover lining provides the advantage of not copying the image on the back side, but suffers from ground fog.